During operation of a so-called “push-feed firearm” an ammunition cartridge is substantially unguided in its travel between the magazine and the chamber after leaving the magazine's feed lips which otherwise retain the cartridges within the magazine. Push-feed firearms contrast with controlled-feed firearms in that during operation of a controlled-feed firearm the cartridge is held first by the magazine feed lips and then held by the extractor as the cartridge is chambered, thereby ensuring proper and reliable ammunition feed during cycling of the firearm action.
The substantially unguided nature of ammunition feed in the push-feed firearm allows a cartridge to be pushed laterally out of alignment with the bolt and chamber by cartridges following the cartridge being chambered as well as by the magazine follower when the cartridge is the last round in the magazine. This effect can be particularly troublesome in prior art rotary magazines 10, as shown in FIG. 1, when feeding the last cartridge 12 in the magazine. As shown in the cross section of FIG. 1A, the follower paddle 14 rotates counterclockwise (in this view) when advancing the cartridges 12, and the natural effect of this rotary motion is to push the cartridge toward the left (in this view) once it is clear of the feed lips 16 and 18. Mis-feeds often result when the paddle 14 kicks the cartridge head off of the bolt. The cartridge is then caught between the advancing bolt head and the front wall of the ejection port. Shorter and relatively lightweight cartridges are especially prone to this type of feed failure. This problem is also encountered in the case of feeding cartridges prior to the last round due to lateral force imparted from the cartridges advanced by the follower and rising under the feeding cartridge.
Prior art push-feed magazines are also prone to other failure modes. For example, a cartridge can simply drop from the action on its way into the chamber if the rifle is held on its side with the ejection port down. There is clearly an opportunity for improvement in magazines for push-feed firearms.